Method and apparatus for mapping a logical address between memories of a storage drive based on write frequency rankings

ABSTRACT

A storage drive including a first and second memories and a controller. The second memory has a write cycle lifetime that is less than a write cycle lifetime of the first memory. Each of the first and second memories includes solid-state memory. The controller: determines a write frequency for a first logical address; and based on the write frequency, determines a write frequency ranking for the first logical address. The write frequency ranking is based on a weighted time-decay average of write counts or an average of elapsed times of write cycles. The controller also: determines whether the write frequency ranking is greater than a lowest write frequency ranking of logical addresses of the first memory; and if the write frequency ranking of the first logical address is greater, maps the logical address with the lowest write frequency ranking in the first memory to the second memory.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/365,455 filed on Feb. 4, 2009, which claims the benefit of U.S.Provisional Application No. 61/032,774 filed on Feb. 29, 2008 and is acontinuation-in-part of U.S. application Ser. No. 11/952,648 filed onDec. 7, 2007, which claims the benefit of U.S. Provisional ApplicationNo. 60/869,493 filed on Dec. 11, 2006. The disclosures of the aboveapplications are incorporated herein by reference in their entirety.

FIELD

The present disclosure relates to solid state memories, and moreparticularly to hybrid nonvolatile solid state memories.

BACKGROUND

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent it is described in thisbackground section, as well as aspects of the description that may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presentdisclosure.

Flash memory chips, which use charge storage devices, have become adominant chip type for semiconductor-based mass storage devices. Thecharge storage devices are particularly suitable in applications wheredata files to be stored include music and image files. Charge storagedevices, however, can sustain a limited number of write cycles afterwhich the charge storage devices can no longer reliably store data.

A limited number of write cycles may be acceptable for many applicationssuch as removable USB (universal serial bus) drives, MP3 (MPEG Layer 3)players, and digital camera memory cards. However, when used as generalreplacements for bulk nonvolatile storage in computer systems, a limitednumber of write cycles may not be acceptable.

Lower density flash devices, where a single bit is stored per storagecell, typically have a usable lifetime on the order of 100,000 writecycles. To reduce cost, flash devices may store 2 bits per storage cell.Storing 2 bits per storage cell, however, may reduce the usable lifetimeof the device to a level on the order of 10,000 write cycles.

Flash devices may not have a long enough lifetime to serve as massstorage, especially where part of the mass storage is used as virtualmemory paging space. Virtual memory paging space is typically used byoperating systems to store data from RAM (random access memory) whenavailable space in RAM is low. For purposes of illustration only, aflash memory chip may have a capacity of 2 GB (gigabytes), may store 2bits per cell, and may have a write throughput of about 4 MB/s(megabytes per second). In such a flash memory chip, it is theoreticallypossible to write every bit in the chip once every 500 seconds (i.e.,2E9 bytes/4E6 bytes/s).

It is then theoretically possible to write every bit 10,000 times inonly 5E6 seconds (1E4 cycles*5E2 seconds), which is less than twomonths. In reality, however, most drive storage will not be written with100% duty cycle. A more realistic write duty cycle may be 10%, which mayhappen when a computer is continuously active and performs virtualmemory paging operations. At 10% write duty cycle, the usable lifetimeof the flash device may be exhausted in approximately 20 months. Bycontrast, the life expectation for a magnetic hard disk storage devicetypically exceeds 10 years.

FIG. 1 illustrates a functional block diagram of a conventionalsolid-state disk 100. The solid-state disk 100 includes a controller 102and a flash memory 104. The controller 102 receives instructions anddata from a host (not shown). When a memory access is requested, thecontroller 102 reads or writes data to the flash memory 104, andcommunicates this information to the host.

An area (or memory block) of the flash memory 104 may become unreliablefor storage after the area has been written to or erased a predeterminednumber of times. This predetermined number of times is referred to asthe write cycle lifetime of the flash memory 104. Once the write cyclelifetime of the flash memory 104 has been exceeded, the controller 102can no longer reliably store data in the flash memory 104, and thesolid-state disk 100 may no longer be usable.

SUMMARY

In various embodiments, the present disclosure is directed to a solidstate memory system. The system comprises a first nonvolatilesemiconductor memory having a first write cycle lifetime and a first setof physical addresses, and a second nonvolatile semiconductor memoryhaving a second write cycle lifetime and a second set of physicaladdresses. The first write cycle lifetime is greater than the secondwrite cycle lifetime. The system further comprises a fatigue managementmodule to generate a write frequency ranking for a plurality of logicaladdresses. The fatigue management module maps each of the plurality oflogical addresses to a physical address of the first set of physicaladdresses or the second set of physical addresses based on the writefrequency rankings.

In various embodiments, the present disclosure is directed to a fatiguemanagement method for a solid state memory system. The method comprisesproviding a first nonvolatile semiconductor memory having a first writecycle lifetime and a first set of physical addresses, and providing asecond nonvolatile semiconductor memory having a second write cyclelifetime and a second set of physical addresses. The first write cyclelifetime is greater than the second write cycle lifetime. The methodfurther comprises generating a write frequency ranking for a pluralityof logical addresses, and mapping each of the plurality of logicaladdresses to a physical address of the first set of physical addressesor the second set of physical addresses based on the write frequencyrankings.

In still other features, the systems and methods described above areimplemented by a computer program executed by one or more processors.The computer program can reside on a computer readable medium such asbut not limited to memory, nonvolatile data storage and/or othersuitable tangible storage mediums.

In still other features, the systems and methods described above areimplemented by a computer program executed by one or more processors.The computer program can reside on a computer readable medium such asbut not limited to memory, nonvolatile data storage and/or othersuitable tangible storage mediums.

Further areas of applicability of the present disclosure will becomeapparent from the detailed description provided hereinafter. It shouldbe understood that the detailed description and specific examples, whileindicating the preferred embodiment of the disclosure, are intended forpurposes of illustration only and are not intended to limit the scope ofthe disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will become more fully understood from thedetailed description and the accompanying drawings, wherein:

FIG. 1 is a functional block diagram of a solid state disk driveaccording to the prior art;

FIG. 2 is a functional block diagram of a solid state disk driveaccording to the present disclosure;

FIG. 3 is a functional block diagram of a solid state disk driveincluding a wear leveling module;

FIG. 4A is a functional block diagram of a solid state disk driveincluding the wear leveling module of FIG. 3 and a write monitoringmodule;

FIG. 4B is a functional block diagram of a solid state disk driveincluding the wear leveling module FIG. 3 and a write mapping module;

FIG. 5 is a functional block diagram of a solid state disk driveincluding a degradation testing module and the wear leveling module ofFIG. 3;

FIG. 6 is a functional block diagram of a solid state disk driveincluding a mapping module and the wear leveling module of FIG. 3;

FIGS. 7A-7E are exemplary flowcharts of a method for operating the solidstate disk drives illustrated in FIGS. 2-5;

FIG. 8 is an exemplary flowchart of a method for operating the solidstate disk drive illustrated in FIG. 6;

FIG. 9 is a functional block diagram of a system including a solid statedisk drive;

FIG. 10 is a functional block diagram of a solid state disk drivecomprising a fatigue management module according to the presentdisclosure;

FIG. 11 is a functional block diagram of a solid state disk drivecomprising a fatigue management module with independent wear levelingmodules for each of the memories according to the present disclosure;

FIG. 12 is an exemplary mapping/write frequency ranking table accordingto the present disclosure; and

FIG. 13 is an exemplary flowchart of a method for operating the solidstate disk drive of FIG. 10.

DETAILED DESCRIPTION

The following description is merely exemplary in nature and is in no wayintended to limit the disclosure, its application, or uses. For purposesof clarity, the same reference numbers will be used in the drawings toidentify similar elements. As used herein, the phrase at least one of A,B, and C should be construed to mean a logical (A or B or C), using anon-exclusive logical or. It should be understood that steps within amethod may be executed in different order without altering theprinciples of the present disclosure. As used herein, the term “basedon” or “substantially based on” refers to a value that is a function of,proportional to, varies with, and/or has a relationship to anothervalue. The value may be a function of, proportional to, vary with,and/or have a relationship to one or more other values as well.

As used herein, the term module refers to an Application SpecificIntegrated Circuit (ASIC), an electronic circuit, a processor (shared,dedicated, or group) and memory that execute one or more software orfirmware programs, a combinational logic circuit, and/or other suitablecomponents that provide the described functionality.

The cost of charge-storage-based flash devices such as Nitride Read-OnlyMemory (NROM) and NAND flash has been decreasing in recent years. At thesame time, new high-density memory technologies are being developed.Some of these memory technologies, such as phase change memory (PCM),may provide significantly higher write endurance capability thancharge-storage-based flash devices. However, being newer technologies,the storage capacity, access time, and/or cost of these memories may beless attractive than the storage capacity, access time, and/or cost ofthe flash devices.

To combine the longer write cycle lifetime of new memory technologieswith the low cost of traditional technologies, a solid-state memorysystem can be constructed using both types of memory. Large amounts oflow cost memory may be combined with smaller amounts of memory having ahigher write cycle lifetime. The memory having the higher write cyclelifetime can be used for storing frequently changing data, such asoperating system paging data.

FIG. 2 depicts an exemplary solid-state memory system. The solid-statememory system may be used as a solid-state disk in a computer system orother device that may need to store data, e.g., a cell phone, set topbox, automobile component, wireless personal digital assistant (PDA),and the like. For example only, a PCM chip, such as a 2 GB PCM chip, maybe combined with NAND flash devices or NROM flash devices. The writecycle lifetime of PCM memory may soon be of the order of 1E13 writecycles. PCM chips having a write cycle lifetime in excess of 1 E7 writecycles are available. At 1E7 write cycles, a PCM chip has a write cyclelifetime that is 1000 times longer than a 2 bit/cell flash device thatcan endure 1E4 write cycles.

PCM chips may provide faster data throughput than the flash device. Forexample, a PCM chip may provide 100 times faster data throughput thanthe flash device. Even if the PCM chip provides 100 times faster datathroughput than the flash device, the 1000 time greater write cyclelifetime yields an effective write cycle lifetime that is 10 timeslonger than the flash device. For example, at 10% write duty cycle, itwould take 15.9 years to exhaust the lifetime of the PCM chip even ifthe PCM chip provides 100 times faster data throughput than the flashdevice.

FIG. 2 illustrates a functional block diagram of an exemplarysolid-state disk 200 according to one implementation of the presentdisclosure. The solid-state disk 200 includes a controller 202 and firstand second solid-state nonvolatile memories 204, 206. Throughout theremainder of this disclosure, solid-state nonvolatile memories may beimplemented as integrated circuits (IC). The controller 202 receivesaccess requests from a host 220. The controller 202 directs the accessrequests to the first solid-state nonvolatile memory 204 or the secondsolid-state nonvolatile memory 206, as described in greater detailbelow.

For example only, the first solid-state nonvolatile memory 204 mayinclude relatively inexpensive nonvolatile memory arrays and have alarge capacity. The second solid-state nonvolatile memory 206 may have agreater write cycle lifetime while being more expensive and having asmaller capacity than the first solid-state nonvolatile memory 204. Invarious implementations, the host 220 may specify to the controller 202the logical addresses that correspond to data that will changerelatively frequently and the logical addresses that correspond to datathat will change relatively infrequently.

The controller 202 may map the logical addresses corresponding to datathat will change relatively frequently to physical addresses in thesecond solid-state nonvolatile memory 206. The controller 202 may mapthe logical addresses corresponding to data that will change relativelyinfrequently to physical addresses in the first solid-state nonvolatilememory 204.

The first solid-state nonvolatile memory 204 may include single-levelcell (SLC) flash memory or multi-level cell (MLC) flash memory. Thesecond solid-state nonvolatile memory 206 may include single-level cell(SLC) flash memory or multi-level cell (MLC) flash memory.

Before a detailed discussion, a brief description of drawings ispresented. FIG. 3 depicts an exemplary solid-state disk 250 including awear leveling module 260. In one implementation, the wear levelingmodule 260 controls mapping between logical addresses from the host 220to physical addresses in the first and second solid-state memories 204,206. The wear leveling module 260 may perform this mapping based oninformation from the host.

Alternatively or additionally, the wear leveling module 260 may measureor estimate the wear across all areas of both the solid-statenonvolatile memories 204, 206 and change the mapping to equalize wearacross all areas of both the solid-state nonvolatile memories 204, 206.In one implementation, the goal of the wear leveling module 260 is tolevel the wear across all the areas of the solid-state nonvolatilememories 204, 206 so that no one area wears out before other areas ofthe solid-state nonvolatile memories 204, 206.

With various nonvolatile memories, writing data to a memory block mayrequire erasing or writing to the entire memory block. In such ablock-centric memory, the wear leveling module 260 may track the numberof times that each memory block has been erased or written. When a writerequest arrives from the host, the wear leveling module 260 may select amemory block of memory that has been written to the least from among theavailable memory blocks. The wear leveling module 260 then maps theincoming logical address to the physical address of this memory block.Over time, this may produce a nearly uniform distribution of writeoperations across memory blocks.

FIGS. 4A and 4B include additional modules that help to control wearleveling. In FIG. 4A, the wear leveling module 260 determines howfrequently data is written to each of the logical addresses. In oneimplementation, logical addresses that are the target of relativelyfrequent writes or erases are mapped to physical addresses that have notexperienced as much wear.

In FIG. 4B, a write mapping module 356 receives write frequencyinformation from the host 220. The write frequency informationidentifies the logical addresses that correspond to data that isexpected to change relatively frequently and/or the logical addressesthat correspond to data that is expected to change relativelyinfrequently. In addition, the write mapping module 356 may determinehow frequently data is actually written to the logical addresses, as inFIG. 4A. FIG. 5 shows a solid-state disk 400 where degradation of thememory and resulting remaining life is determined empirically, inaddition to or instead of estimating remaining life based on the numberof writes or erases.

FIG. 6 shows a solid-state disk 450 where a combination of first andsecond solid-state nonvolatile memories 462, 464 is used for cachingdata. In one implementation, the first solid-state nonvolatile memory462 has a high storage capacity (for example, 2 GB or greater). In oneimplementation, the second solid-state nonvolatile memory 464 has afaster access time than the first solid-state nonvolatile memory 462,and has a smaller storage capacity (for example, 2 GB or less) than thefirst solid-state nonvolatile memory 462. The first and second memories462, 464 may both have high write cycle lifetimes.

A mapping module 465 may be used to map logical addresses from a host tothe first and second solid-state nonvolatile memories 462, 464 based onaccess time considerations. The mapping module 465 may receive accesstime information from the host 480, such as a list of addresses forwhich quick access times are or are not desirable. Alternatively oradditionally, the mapping module 465 may monitor accesses to logicaladdresses, and determine for which logical addresses reduced accesstimes would be most beneficial. The logical addresses for which lowaccess times are important may be mapped to the second solid-statenonvolatile memory 464, which (in one implementation) has reduced accesstimes.

As used herein, access times may include, for example, read times, writetimes, erase times, and/or combined access times that incorporate one ormore of the read, write, or erase times. For example, a combined accesstime may be an average of the read, write, and erase times. By directingcertain logical addresses to be mapped to the second solid-statenonvolatile memory 464, the host 480 may optimize storage for operationssuch as fast boot time or application startup. The mapping module 465may also be in communication with a wear leveling module 260 that adaptsthe mapping to prevent any one area in the first and second solid-statenonvolatile memories 462, 464 from wearing out prematurely.

FIGS. 7A-7E depict exemplary steps performed by the controllers shown inFIGS. 4A-5. FIG. 8 depicts exemplary steps performed by the controllershown in FIG. 6. A detailed discussion of the systems and methods shownin FIGS. 2-8 is now presented.

Referring now to FIG. 3, a solid-state disk 250 includes a controller252 and the first and second solid-state nonvolatile memories 204 and206. The controller 252 communicates with the host 220. The controller252 comprises a wear leveling module 260 and first and second memoryinterfaces 262, 264. The wear leveling module 260 communicates with thefirst and second solid-state nonvolatile memory 204 via first and secondmemory interfaces 262, 264, respectively.

The wear leveling module 260 receives logical addresses from the host220. The logical addresses are converted into physical addressesassociated with the first memory interface 262 and/or the second memoryinterface 264. During a write operation, data from the host 220 iswritten to the first solid-state nonvolatile memory 204 via the firstmemory interface 262 or to the second solid-state nonvolatile memory 206via the second memory interface 264. During a read operation, data isprovided to the host 220 from the first or second solid-statenonvolatile memory 204, 206 via the first or second memory interface262, 264, respectively.

For example only, the first solid-state nonvolatile memory 204 may berelatively inexpensive per megabyte of capacity and may therefore have alarge capacity. The second solid-state nonvolatile memory 206 may have alonger write cycle lifetime and may be more expensive than the firstsolid-state nonvolatile memory 204, and may therefore have a smallercapacity.

The first and second solid-state nonvolatile memories 204 and 206 may bewritten to and/or erased in blocks. For example, in order to erase onebyte in a memory block, all bytes of the memory block may need to beerased. In addition, in order to write one byte of a memory block, allbytes of the memory block may need to be written. The wear levelingmodule 260 may track and store the number of write and/or eraseoperations performed on the memory blocks of the first and secondsolid-state nonvolatile memories 204, 206.

The wear leveling module 260 may use a normalized version of the writeand/or erase cycle counts. For example, the number of write cyclesperformed on a memory block in the first solid-state nonvolatile memory204 may be divided by the total number of write cycles that a memoryblock in the first solid-state nonvolatile memory 204 can endure. Anormalized write cycle count for a memory block in the secondsolid-state nonvolatile memory 206 may be obtained by dividing thenumber of write cycles already performed on that memory block by thenumber of write cycles that the memory block can endure.

The wear leveling module 260 may write new data to the memory block thathas the lowest normalized write cycle count. To avoid fractional writecycle counts, the write cycle counts can be normalized by multiplyingthe write cycle counts by constants based on the write cycle lifetime ofthe respective memories 204 and 206. For example, the number of writecycles performed on a memory block of the first solid-state nonvolatilememory 204 may be multiplied by a ratio. The ratio may be the writecycle lifetime of the second solid-state nonvolatile memory 206 dividedby the write cycle lifetime of the first solid-state nonvolatile memory204.

In various implementations, the write cycle count may only be partiallynormalized. For example, the write cycle lifetime of the secondsolid-state nonvolatile memory 206 may be significantly higher than thewrite cycle lifetime of the first solid-state nonvolatile memory 204. Insuch a case, the write cycle count of the first solid-state nonvolatilememory 204 may be normalized using a write cycle lifetime that is lessthan the actual write cycle lifetime. This may prevent the wear levelingmodule 260 from being too heavily biased toward assigning addresses tothe second solid-state nonvolatile memory 206.

The normalization may be performed using a predetermined factor. Forexample, if the write cycle lifetime of the first solid-statenonvolatile memory 204 is 1 E6, and for a given application of thesolid-state disk 250, the necessary write cycle lifetime of the secondsolid-state nonvolatile memory 206 is 1 E9, the normalization can beperformed using a factor of 1,000. The factor may be a rounded offestimate and not an exact calculation. For example, a factor of 1000 maybe used when respective write cycle lifetimes are 4.5 E6 and 6.3 E9.

The wear leveling module 260 may include a data shifting module 261. Inone implementation, the data shifting module 261 identifies a firstmemory block having stored data that remains unchanged over apredetermined period of time. Such stored data may be called staticdata. The static data may be moved to a second memory block of memorythat has experienced more frequent write cycles than the first memoryblock. The wear leveling module 260 may map the logical addresses thatwere originally mapped to the physical addresses of the first memoryblock, to the physical addresses of the second memory block. Since thestatic data is now stored in the second memory block, the second memoryblock may experience fewer write cycles.

Additionally, static data may be shifted from the second solid-statenonvolatile memory 206 to the first solid-state nonvolatile memory 204.For example, the data shifting module 261 may identify a least usedmemory block (LUB) of the second solid-state nonvolatile memory 206. Ifa number of write operations performed on a memory block during apredetermined period is less than or equal to a predetermined threshold,the memory block is called a LUB. When the amount of usable or availablememory in the second solid-state nonvolatile memory 206 decreases to apredetermined threshold, the wear leveling module 260 may map the LUB toa memory block of the first solid-state nonvolatile memory 204.

Occasionally, the number of write operations performed on a first memoryblock of the first solid-state nonvolatile memory 204 may exceed apredetermined threshold. The wear leveling module 260 may bias mappingof logical addresses that were originally mapped to the first memoryblock, to a second memory block of the second solid-state nonvolatilememory 206 thereby reducing the wear on the first solid-statenonvolatile memory 204.

Referring now to FIG. 4A, a solid-state disk 300 includes a controller302 that interfaces with the host 220. The controller 302 includes thewear leveling module 260, a write monitoring module 306, and the firstand second memory interfaces 262 and 264. The write monitoring module306 monitors logical addresses received from the host 220. The writemonitoring module 306 may also receive control signals indicatingwhether a read or a write operation is occurring. Additionally, thewrite monitoring module 306 tracks the logical addresses to which datais frequently written by measuring frequencies at which data is writtento the logical addresses. This information is provided to the wearleveling module 260, which biases the logical addresses to, e.g.,physical addresses of the second solid-state nonvolatile memory 206.

Referring now to FIG. 4B, a solid-state disk 350 includes a controller352, which interfaces with the host 220. The controller 352 includes awear leveling module 260, a write mapping module 356, and first andsecond memory interfaces 262, 264. The write mapping module 356 receivesaddress information from the host 220 indicating the logical addressesthat will be more frequently written to. This information is provided tothe wear leveling module 260, which biases the logical addresses to thesecond solid-state nonvolatile memory 206.

The write mapping module 356 may also include functionality similar tothe write monitoring module 306 of FIG. 4A. The write mapping module 356may therefore update stored write frequency data based on measured writefrequency data. Additionally, the write mapping module 356 may determinewrite frequencies for the logical addresses that were not provided bythe host 220. In one implementation, the write frequency data may beadjusted even if a logical address has not been accessed for apredetermined period. The wear leveling module 260 may store all datacorresponding to the logical addresses that are flagged as frequentlywritten to in the second solid-state nonvolatile memory 206.

If the second solid-state nonvolatile memory 206 is full, the writeoperations may be assigned to the first solid-state nonvolatile memory204 and vice versa. Data can also be remapped and moved from the secondsolid-state nonvolatile memory 206 to the first solid-state nonvolatilememory 204 to create space in the second solid-state nonvolatile memory206 and vice versa. Alternatively, data may be mapped solely to thefirst or the second solid-state nonvolatile memory 204, 206 when thewear level of the second or the first solid-state nonvolatile memory206, 204 is greater than or equal to a predetermined threshold. Itshould be noted that the predetermined threshold for the wear level ofthe first and second solid-state nonvolatile memory 204, 206 may be thesame or different. Furthermore, the predetermined threshold may vary atdifferent points in time. For example, once a certain number of writeoperations have been performed on the first solid-state nonvolatilememory 204, the predetermined threshold may be adjusted to take intoconsideration the performed write operations.

The wear leveling module 260 may also implement the write monitoringmodule 306 and the write mapping module 356. Hereinafter, the wearleveling module 260 may also include the write monitoring module 306 andthe write mapping module 356.

Referring now to FIG. 5, the solid-state disk 400 includes a controller402 that interfaces with the host 220. The controller 402 includes thewear leveling module 260, a degradation testing module 406, and thefirst and second memory interfaces 262, 264. The degradation testingmodule 406 tests the first and second solid-state nonvolatile memories204, 206 to determine whether their storage capability has degraded.

In various implementations, the degradation testing module 406 may testonly the first solid-state nonvolatile memory 204, since the write cyclelifetime of the first solid-state nonvolatile memory 204 (in oneimplementation) is less than the write cycle lifetime of the secondsolid-state nonvolatile memory 206. The degradation testing module 406may periodically test for degradation. The degradation testing module406 may wait for periods of inactivity, at which point the degradationtesting module 406 may provide addresses and data to the first and/orsecond memory interfaces 262, 264.

The degradation testing module 406 may write and then read data toselected areas of the first and/or second solid-state nonvolatilememories 204, 206. The degradation testing module 406 can then comparethe read data to the written data. In addition, the degradation testingmodule 406 may read data written in previous iterations of degradationtesting.

Alternatively, the degradation testing module 406 may write the samedata to the same physical address at first and second times. At each ofthe two times, the degradation testing module 406 may read back the datawritten. The degradation testing module 406 may determine a degradationvalue for the physical address by comparing the data read back at thetwo times or by comparing the data read back at the second time to thewritten data.

The wear leveling module 260 may adapt its mapping based on thedegradation value measured by the degradation testing module 406. Forexample, the degradation testing module 406 may estimate a maximum writecycle count for a memory block based on the amount of degradation. Thewear leveling module 260 may then use this maximum write cycle count fornormalization.

Alternatively, the wear leveling module 260 may use the number of writescycles remaining for a memory block to make assignment decisions. If oneof the solid-state nonvolatile memories 204, 206 is approaching the endof its usable lifetime (e.g., a predetermined threshold), the wearleveling module 260 may assign all new writes to the other one of thesolid-state nonvolatile memories 204, 206.

The wear leveling module 260 may also implement the degradation testingmodule 406. Hereinafter, the wear leveling module 260 includes thedegradation testing module 406.

Referring now to FIG. 6, a small solid-state nonvolatile memory having afaster access time may be used in combination with a large solid-statenonvolatile memory having a slower access time. A solid-state disk 450may include a controller 460, a first solid-state nonvolatile memory462, and a second solid-state nonvolatile memory 464. The controller 460interfaces with a host 480. The controller 460 may include first andsecond memory interfaces 472, 474. The first solid-state nonvolatilememory 462 may be inexpensive and may have a high storage capacity and ahigh write cycle lifetime but a lower read/write speed (i.e., accesstime). The second solid-state nonvolatile memory 464 may be smaller instorage capacity, may be more expensive, and may have a high write cyclelifetime and a faster access time relative to the first solid-statenonvolatile memory 462.

The second solid-state nonvolatile memory 464 may have a write accesstime, a read access time, an erase time, a program time, or a cumulativeaccess time that is shorter than that of the first solid-statenonvolatile memory 462. Accordingly, the second solid-state nonvolatilememory 464 may be used to cache data. The controller 460 may include thewear leveling module 260 and a mapping module 465. The wear levelingmodule 260 may also implement the mapping module. The mapping module 465may map the logical addresses to the physical addresses of one of thefirst and second solid-state nonvolatile memory 462, 464 based on accesstimes and/or storage capacities of the first and second solid-statenonvolatile memory 462, 464.

Specifically, the mapping module may receive data from the host 220related to the frequencies and access times at which data may be writtento the logical addresses. The mapping module 465 may map the logicaladdresses that are to be written more frequently and/or faster thanothers to the physical addresses of second solid-state nonvolatilememory 464. All other logical addresses may be mapped to the physicaladdresses of the first nonvolatile memory 462. The actual writefrequencies access times may be updated by measuring write frequenciesand/or access times when data is written. In doing so, the mappingmodule 465 may minimize overall access time for all accesses made to thesolid-state disk 450 during read/write/erase operations.

Depending on the application executed by the host 220, the mappingmodule 465 may consider additional factors when mapping the logicaladdresses to one of the first and second solid-state nonvolatile memory462, 464. The factors may include but are not limited to the length of amemory block being written and the access time with which the memoryblock needs to be written.

Referring now to FIGS. 7A-7E, a method 500 for providing a hybridnonvolatile solid-state (NVS) memory system using first and second NVSmemories having different write cycle lifetimes and storage capacitiesis shown. The first NVS memory has a lower write cycle lifetime andhigher capacity than the second NVS memory.

In FIG. 7A, the method 500 begins at step 502. Control receives writefrequencies for logical addresses where data is to be written from thehost in step 504. Control maps the logical addresses having low writefrequencies (e.g., having write frequencies less than a predeterminedthreshold) to the first NVS memory in step 506. Control maps the logicaladdresses having high write frequencies (e.g., having write frequenciesgreater than a predetermined threshold) to the second NVS memory in step508.

Control writes data to the first and/or second NVS memories in step 510according to the mapping generated in steps 506 and 508. Controlmeasures actual write frequencies at which data is in fact written tothe logical addresses and updates the mapping in step 512.

In FIG. 7B, control determines whether time to perform data shiftanalysis has arrived in step 514. If the result of step 514 is false,control determines whether time to perform degradation analysis hasarrived in step 516. If the result of step 516 is false, controldetermines whether time to perform wear level analysis has arrived instep 518. If the result of step 514 is false, control returns to step510.

In FIG. 7C, when the result of step 514 is true, control determines instep 520 if a number of write operations to a first memory block of thefirst NVS memory during a predetermined time is greater than or equal toa predetermined threshold. If the result of step 520 is false, controlreturns to step 516. If the result of step 520 is true, control maps thelogical addresses that correspond to the first memory block to a secondmemory block of the second NVS memory in step 522.

Control determines in step 524 if the available memory in the second NVSmemory is less than a predetermined threshold. If the result of step 524is false, control returns to step 516. If the result of step 524 istrue, control identifies a memory block of the second NVS memory is aLUB in step 526. Control maps the logical addresses that correspond tothe LUB to a memory block of the first NVS memory in step 528, andcontrol returns to step 516.

In FIG. 7D, when the result of step 516 is true, control writes data toa physical address at a first time in step 530. Control reads back thedata from the physical address in step 532. Control writes data to thephysical address at a second time (e.g., after a predetermined timeafter the first time) in step 534. Control reads back the data from thephysical address in step 536. Control compares the data read back instep 532 to the data read back in step 536 and generates a degradationvalue for the physical address in step 538. Control updates the mappingin step 540, and control returns to step 518.

In FIG. 7E, when the result of step 518 is true, control generates wearlevels for the first and second NVS memories in step 542 based on thenumber of write operations performed on the first and second memoriesand the write cycle lifetime ratings of the first and second memories,respectively. Control determines in step 544 if the wear level of thesecond NVS memory is greater than a predetermined threshold. If theresult of step 544 is true, control maps all the logical memory blocksto physical memory blocks of the first NVS memory in step 546, andcontrol returns to step 510.

If the result of step 544 is false, control determines in step 548 ifthe wear level of the first NVS memory is greater than a predeterminedthreshold. If the result of step 548 is true, control maps all thelogical memory blocks to physical memory blocks of the second NVS memoryin step 550 and, control returns to step 510. If the result of step 548is false, control returns to step 510.

Referring now to FIG. 8, a method 600 for providing a hybrid nonvolatilesolid-state (NVS) memory system for caching data using first and secondNVS memories having different access times and storage capacities isshown. The first NVS memory has a higher access time and higher capacitythan the second NVS memory. The first and second NVS memories have highwrite cycle lifetimes.

The method 600 begins at step 602. Control receives data related towrite frequency and access time requirement for writing data to logicaladdresses from the host in step 604. Control maps the logical addresseshaving low write frequencies (e.g., having write frequencies less than apredetermined threshold) and/or requiring slower access times to thefirst NVS memory in step 606. Control maps the logical addresses havinghigh write frequencies (e.g., having write frequencies greater than apredetermined threshold) and/or requiring faster access times to thesecond NVS memory in step 606. Control maps the logical addresses havinglow write frequencies (e.g., having write frequencies less than apredetermined threshold) and/or requiring slower access times to thefirst NVS memory in step 608.

Control writes data to the first and/or second NVS memories in step 610according to the mapping generated in steps 606 and 608. Controlmeasures actual write frequencies and/or actual access times at whichdata is in fact written to the logical addresses and updates the mappingin step 612. In step 614, control executes steps beginning at step 514of the method 500 as shown in FIGS. 7A-7E.

Wear leveling modules according to the principles of the presentdisclosure may determine wear levels for each memory block of one ormore nonvolatile semiconductor memories. The term memory block may referto the group of memory cells that must be written and/or erasedtogether. For purposes of discussion only, the term memory block will beused for a group of memory cells that is erased together, and the wearlevel of a memory cell will be based on the number of erase cycles ithas sustained.

The memory cells within a memory block will have experienced the samenumber of erases, although individual memory cells may not have beenprogrammed when the erase was initiated, and thus may not experience asmuch wear. However, the wear leveling module may assume that the wearlevels of the memory cells of a memory block can be estimated by thenumber of erase cycles the memory block has experienced.

The wear leveling module may track the number of erases experienced byeach memory block of the first and second memories. For example, thesenumbers may be stored in a certain region of the first and/or secondmemories, in a separate working memory of the wear leveling module, orwith their respective memory blocks. For example only, a predeterminedarea of the memory block, which is not used for user data, may be usedto store the total number of times that memory block has been erased.When a memory block is going to be erased, the wear leveling module mayread that value, increment the value, and write the incremented value tothe memory block after the memory block has been erased.

With a homogeneous memory architecture, the erase count could be used asthe wear level of a memory block. However, the first and second memoriesmay have different lifetimes, meaning that the number of erases eachmemory cell can withstand is different. In various implementations, thesecond memory has a longer lifetime than the first memory. The number oferases each memory block can withstand is therefore greater in thesecond memory than in the first.

The number of erases performed on a memory block may therefore not be anappropriate comparison between a memory block from the first memory anda memory block of the second memory. To achieve appropriate comparisons,the erase counts can be normalized. One way of normalizing is to dividethe erase count by the total number of erase counts a memory block inthat memory is expected to be able to withstand. For example only, thefirst memory has a write cycle lifetime of 10,000, while the secondmemory has a write cycle lifetime of 100,000.

A memory block in the first memory that has been erased 1,000 timeswould then have a normalized wear level of 1/10, while a memory block inthe second memory that has been erased 1,000 times would then have anormalized wear level of 1/100. Once the wear levels have beennormalized, a wear leveling algorithm can be employed across all thememory blocks of both the first and second memories as if all the memoryblocks formed a single memory having a singe write cycle lifetime. Wearlevels as used herein, unless otherwise noted, are normalized wearlevels.

Another way of normalizing, which avoids fractional numbers, is tomultiply the erase counts of memory blocks in the first memory (havingthe lower write cycle lifetime) by the ratio of write cycle lifetimes.In the current example, the ratio is 10 (100,000/10,000). A memory blockin the first memory that has been erased 1,000 times would then have anormalized wear level of 10,000, while a memory block in the secondmemory that has been erased 1,000 times would then have a normalizedwear level of 1,000.

When a write request for a logical address arrives at the wear levelingmodule, the wear leveling module may determine if the logical address isalready mapped to a physical address. If so, the wear leveling modulemay direct the write to that physical address. If the write wouldrequire an erase of the memory block, the wear leveling module maydetermine if there are any unused memory blocks with lower wear levels.If so, the wear leveling module may direct the write to the unusedmemory block having the lowest wear level.

For a write request to a logical address that is not already mapped, thewear leveling module may map the logical address to the unused memoryblock having the lowest wear level. If the wear leveling module expectsthat the logical address will be rewritten relatively infrequently, thewear leveling module may map the logical address to the unused memoryblock having the highest wear level.

When the wear leveling module has good data for estimating accessfrequencies, the wear leveling module may move data from a used memoryblock to free that memory block for an incoming write. In this way, anincoming write to a memory block that is relatively frequently accessedcan be written to a memory block with a low wear level. Also, anincoming write to a memory block that is relatively infrequentlyaccessed can be written to a memory block with a high wear level. Thedata that was moved can be placed in an unused memory block that may bechosen based on how often the moved data is expected to be rewritten.

At various times, such as periodically, the wear leveling module mayanalyze the wear levels of the memory blocks, and remap relativelyfrequently rewritten logical addresses to memory blocks with low wearlevels. In addition, the wear leveling module may remap relativelyinfrequently rewritten logical addresses to memory blocks with high wearlevels, which is known as static data shifting. Remapping may involveswapping data in two memory blocks. During the swap, the data from oneof the memory blocks may be stored in an unused memory block, or intemporary storage.

The wear leveling module may also maintain a list of memory blocks thathave surpassed their write cycle lifetime. No new data will be writtento these memory blocks, and data that was previously stored in thosememory blocks is written to other memory blocks. Although the goal ofthe wear leveling module is that no memory block wears out before theothers, some memory blocks may wear out prematurely under real-worldcircumstances. Identifying and removing unreliable memory blocks allowsthe full lifetime of the remaining memory blocks to be used before thesolid-state disk is no longer usable.

It should be understood that while the present disclosure, forillustration purposes, describes first and second solid-statenonvolatile memories 204, 206, the teachings of the present disclosuremay also be applied to other types of memories. In addition, thememories may not be limited to individual modules. For example, theteachings of the present disclosure may be applied to memory zoneswithin a single memory chip or across multiple memory chips. Each memoryzone may be used to store data in accordance with the teachings of thepresent disclosure.

FIG. 9 illustrates a system 900. The system 900 can be any device thatstores data, e.g., a computer, set top box, cellular phone (or othertype of wireless handheld device), and the like. The system 900 includessolid state disk 200 for storing data as described above.

FIG. 10 illustrates an exemplary solid-state memory system according tothe present disclosure. The memory system includes a solid-state disk1000 in communication with a host 1005. The solid-state disk maycomprise a controller 1010, a first solid-state nonvolatile memory 1001and a second solid-state nonvolatile memory 1002. As an example only,the first solid-state nonvolatile memory 1001 may comprise a highendurance (i.e., high write cycle lifetime) memory device, such as asingle-level cell (SLC) flash chip. The first solid-state nonvolatilememory 1001 may be more expensive and have a lower capacity (and/ordensity) as compared to second solid-state nonvolatile memory 1002,which may comprise a lower endurance and/or higher capacity (and/ordensity) memory device, such as a multi-level cell (MLC) flash chip. Inthis manner, solid-state disk 1000 may provide a storage system thatbalances endurance levels, capacity, and cost.

Controller 1010 may comprise a first memory interface 1011 and a secondmemory interface 1012 for interfacing with the first and secondsolid-state nonvolatile memories, respectively. Further, the controller1010 may include a mapping module 1013 and a fatigue management module1014 for mapping the logical addresses received from host 1005 to thephysical addresses present in the first and second solid-statenonvolatile memories 1001, 1002. During a write operation, data from thehost 1005 is written to the first solid-state nonvolatile memory 1001via the first memory interface 1011 or to the second solid-statenonvolatile memory 1002 via the second memory interface 1012. During aread operation, data is provided to the host 1005 from the first orsecond solid-state nonvolatile memory 1001, 1002 via the first or secondmemory interface 1011, 1012, respectively.

The mapping module 1013 and fatigue management module 1014 may determineto which of the first or second solid-state nonvolatile memories 1001,1002 a particular logical address will be mapped. Fatigue managementmodule 1014 may also incorporate the mapping module, such that itperforms the functions of both the mapping module 1013 and fatiguemanagement module 1014.

In general, the fatigue management module 1014 monitors the numberand/or frequency of write operations to the logical addresses receivedfrom the host 1005. The logical addresses may identify one or morecorresponding memory blocks. Fatigue management module 1014 will map themost frequently written to logical addresses to physical addresses thatreside in the memory with the higher endurance. In furtherance of thisgoal, fatigue management module 1014 may generate a write frequencyranking for each of the logical addresses received from host 1005. Thewrite frequency rankings may comprise the full set of logical addressesin the order of the number of writes to each logical address.Alternatively, the write frequency ranking may comprise the number ofwrite operations to each logical address over a predetermined period.The use of a predetermined period allows for logical addresses that havea high number of total write operations, but which are no longer beingwritten to frequently, to change from being designated as a highfrequency address to a low frequency address.

In order to generate the write frequency ranking, the fatigue managementmodule 1014 may keep a write count for each logical address to which thehost 1005 requests a write operation. When the solid-state disk 1000 isfirst used, for example, when an operating system is first installed, orafter re-formatting in which there is a re-definition of swap space, thefatigue management module 1014 and mapping module 1013 may first writeto the higher endurance nonvolatile memory, which in the illustratedexample is first solid-state nonvolatile memory 1001. After apredetermined percentage or all of the useable physical addresses of thefirst solid-state nonvolatile memory 1001 are populated, the nextwritten logical address may be mapped to the lower endurance nonvolatilememory. Once the higher endurance memory is populated to a certainthreshold, and the lower endurance memory begins being written to,fatigue management module 1014 will operate to manage the mapping to thetwo memories. In other words, the most frequently written to logicaladdresses will be mapped to physical addresses in the higher endurancememory, while the least frequently written to logical addresses will bemapped to the lower endurance memory.

In one example, the fatigue management module 1014 ranks logicaladdresses according to the total number of write operations that havebeen performed to them. The highest ranked logical addresses will bemapped to physical addresses in the higher endurance memory, with theremaining logical addresses being addressed to the physical addresses inthe lower endurance memory. Alternatively, the ranking of logicaladdresses may instead be based upon the number of write operations overa predetermined period. In this example, heavily written to logicaladdresses that have not been written to for a period of time may rankbelow logical addresses that have recently been written to morefrequently. Thus, logical addresses that are mapped to the higherendurance memory may have a lower total number of write operationsassociated with them than logical addresses in the lower endurancememory. In yet another example, a combination of the total writeoperations and a write per period ranking may be utilized.

In another exemplary embodiment, the write frequency for a logicaladdress may be determined by the period of time that has elapsed sincethe logical address was last written to. In this manner, the writefrequency rankings can be determined by putting the most recentlywritten to logical address at the top of the rankings, while the logicaladdress with the longest period of time since being written to will beat the bottom. It is contemplated that the elapsed time may be storedsuch a complete re-ranking of logical addresses does not occur each timethe solid-state disk 1000 is powered on. In yet another embodiment, anaverage of the elapsed time between write cycles for a particularlogical address may be used to generate the write frequency ranking foreach logical address. Thus, a previously frequently written to logicaladdress that has since become infrequently written to will eventuallymigrate to a lower write frequency ranking and, thus, be stored in thelower endurance memory. In a further embodiment, the average of theelapsed time between write cycles may be normalized based on the writecycle lifetimes (or, remaining write cycle lifetimes) of the lower andhigher endurance memories, as is discussed more fully above.

In order to determine a write frequency and, thus, the write frequencyranking for a logical address, a weighted time-decay average of thewrite count for each logical address can be calculated according to thefollowing equation:

WCA(n+1)=WCA(n)*(1−a)+WE(n)*a,  (1)

where WCA(n) is the time averaged write count at timestep n; WE(n) isthe actual write event at timestep n, which equals 1 if there was writeperformed at timestep n or otherwise equals 0; and ‘a’ is a constantchosen to have the appropriate time-decay, where ‘a’ is sometimesreferred to as the “attack rate constant” and (1−a) is sometimesreferred to as the “decay rate constant.” Alternatively, a two parametersystem could be used, such that Equation 1 above becomes:

WCA(n+1)=WCA(n)*d+WE(n)*a,  (2)

where d is the decay rate constant, and all other variables are the sameas above.

The actual mapping of logical addresses to physical addresses withineither the first or second memories may include the wear levelingfunctionality described above. Thus, the assignment of a particularlogical address to either the first or second memory may be determinedby the fatigue management module 1014, while the particular physicaladdress within the selected memory may be determined by the wearleveling module, as described above.

Referring now to FIG. 11, an exemplary solid-state memory systemincluding independent wear leveling modules for each of the memoriesaccording to the present disclosure is illustrated. The memory systemincludes a solid-state disk 1200 in communication with a host 1205. Thesolid-state disk may comprise a controller 1210, a first solid-statenonvolatile memory 1201 and a second solid-state nonvolatile memory1202. As an example only, the first solid-state nonvolatile memory 1201may comprise a high endurance (i.e., high write cycle lifetime) memorydevice, such as a single-level cell (SLC) flash chip. The firstsolid-state nonvolatile memory 1201 may be more expensive and have alower capacity (and/or density) as compared to second solid-statenonvolatile memory 1202, which may comprise a lower endurance and/orhigher capacity (and/or density) memory device, such as a multi-levelcell (MLC) flash chip. In this manner, solid-state disk 1200 may providea storage system that balances endurance levels, capacity, and cost.

Controller 1210 may comprise a first memory interface 1211 and a secondmemory interface 1212 for interfacing with the first and secondsolid-state nonvolatile memories, respectively. Further, the controller1210 may include a mapping module 1213 and a fatigue management module1214 for mapping the logical addresses received from host 1205 to thephysical addresses present in the first and second solid-statenonvolatile memories 1201, 1202. During a write operation, data from thehost 1205 is written to the first solid-state nonvolatile memory 1201via the first memory interface 1211 or to the second solid-statenonvolatile memory 1202 via the second memory interface 1212. During aread operation, data is provided to the host 1205 from the first orsecond solid-state nonvolatile memory 1201, 1202 via the first or secondmemory interface 1211, 1212, respectively.

The mapping module 1213 and fatigue management module 1214 may determineto which of the first or second solid-state nonvolatile memories 1201,1202 a particular logical address will be mapped. Fatigue managementmodule 1214 may also incorporate the mapping module, such that itperforms the functions of both the mapping module 1213 and fatiguemanagement module 1214. The mapping module 1213 may also include firstand second wear leveling modules 1215, 1216. Alternatively, first andsecond wear leveling modules 1215, 1216 may be included in fatiguemanagement module 1214, or may even be separate from mapping module 1213and fatigue management module 1214 (not shown).

The first and second wear leveling modules 1215, 1216 may provide forindependent wear leveling of actual physical addresses within each ofthe first and second solid-state nonvolatile memories 1201, 1202. Asdiscussed above, the goal of the wear leveling module may be to levelthe wear across all the areas of its solid-state nonvolatile memory sothat no one area (or physical address) wears out before the rest of theareas within the memory. An overhead area within each of the first andsecond solid-state nonvolatile memories 1201, 1203 may be utilized formoving data between physical addresses within a memory, which issometimes referred to as “garbage collection.”

Referring now to FIG. 12, an exemplary mapping/write frequency rankingtable 800 according to the present disclosure is shown. Themapping/write frequency ranking table 800 comprises a list of logicaladdresses LA₁-LA_(n) in column 802. For each logical address LA₁-LA_(n),a total write count is monitored and maintained in column 804. The totalwrite counts WC₁-WC_(n) comprises the number of writes performed to thelogical addresses LA₁-LA_(D), respectively. At column 806, the writefrequency rankings WFR₁-WFR_(n) for logical addresses LA₁-LA_(n),respectively, is stored. The write frequency rankings WFR₁-WFR_(n) maybe determined in various ways, as discussed above. The physicaladdresses PA₁-Pa_(n) to which logical addresses LA₁-LA_(n),respectively, have been mapped are stored in column 808. Columns 804-808are updated as necessary or desirable, such that the informationcontained therein is accurate and/or up-to-date.

Referring now to FIG. 13, an exemplary method 1100 for managing thefatigue of a hybrid endurance solid-state storage device is shown. Inthis example, the first memory has a higher endurance level (or writecycle lifetime) than the second NVS memory. The method 1100 begins atstep 1102. Control receives the logical address 1130 to be written to atstep 1104. At step 1106, control determines the write frequency for thereceived logical address 1130. Based on the write frequencydetermination at step 1106, the write frequency ranking for that logicaladdress 1130 is determined at 1108.

At step 1110, the write frequency ranking for the logical address 1130to be written to is compared with the lowest write frequency rankingpresent in the first memory. If the write frequency ranking for thelogical address 1130 is lower than the lowest write frequency ranking inthe first memory, at step 1112 the logical address 1130 is mapped to thesecond memory. At step 1114, the data is written according to themapping, which comprises writing the data to be written to the logicaladdress 1130 to the second memory array. The method 1100 ends at step1116.

If, however, the write frequency ranking for the logical address 1130 ishigher than the lowest write frequency ranking in the first memory, themethod passes to step 1118. At step 1118, the logical address with thelowest write frequency ranking present in the first memory is mappedinstead to the second memory. At step 1120, the data present at thelogical address comprising the lowest write frequency ranking in thefirst memory is moved to the second memory. The logical address 1130 isthen mapped to the first memory at step 1122. The method 1100 thenpasses to step 1114, which writes data according to the mapping. Themethod 1100 ends at step 1116.

Those skilled in the art can now appreciate from the foregoingdescription that the broad teachings of the disclosure can beimplemented in a variety of forms. For example, one or more steps ofmethods described above may be performed in a different order (orconcurrently) and still achieve desirable results. Therefore, while thisdisclosure includes particular examples, the true scope of thedisclosure should not be so limited since other modifications willbecome apparent to the skilled practitioner upon a study of thedrawings, the specification, and the following claims.

What is claimed is:
 1. A storage drive comprising: a first memory; asecond memory having a write cycle lifetime that is less than a writecycle lifetime of the first memory, wherein each of the first memory andthe second memory comprises solid-state memory; and a controllerconfigured to receive a first logical address from a host, wherein thehost is separate from the storage drive, determine a write frequency forthe first logical address, wherein the write frequency indicates howfrequently data is written to the first logical address, based on thewrite frequency, determine a write frequency ranking for the firstlogical address, wherein the write frequency ranking is based on (i) aweighted time-decay average of write counts, or (ii) an average ofelapsed times of write cycles, determine whether the write frequencyranking is greater than a lowest write frequency ranking of logicaladdresses of the first memory, and if the write frequency ranking of thefirst logical address is greater than the lowest write frequency rankingof logical addresses of the first memory, map the logical address withthe lowest write frequency ranking in the first memory to the secondmemory.
 2. The storage drive of claim 1, wherein the controller isconfigured to, when mapping the logical address with the lowest writefrequency ranking in the first memory to the second memory, map thelogical address with the lowest write frequency ranking in the firstmemory from a physical address of the first memory to a physical addressof the second memory.
 3. The storage drive of claim 1, wherein thecontroller is configured to: if the write frequency ranking of the firstlogical address is greater than the lowest write frequency ranking oflogical addresses of the first memory, map the first logical address tothe second memory subsequent to mapping the logical address with thelowest write frequency ranking in the first memory to the second memory;and if the write frequency ranking of the first logical address is lessthan the lowest write frequency ranking of logical addresses of thefirst memory, map the first logical address to the second memory.
 4. Thestorage drive of claim 1, wherein the controller is configured to, ifthe write frequency ranking of the first logical address is greater thanthe lowest write frequency ranking of logical addresses of the firstmemory, move data at the logical address of the first memory with thelowest write frequency ranking to the second memory.
 5. The storagedrive of claim 1, wherein the write frequency ranking for the firstlogical address is based on the weighted time-decay average of writecounts.
 6. The storage drive of claim 5, wherein the weighted time-decayaverage of write counts is based on a sum of (i) a product of a firstconstant and a time averaged write count at a timestep, and (ii) aproduct of a second constant and a write event value.
 7. The storagedrive of claim 6, wherein the write event value is equal to (i) a firstvalue if a write event occurred at the timestep, and (ii) a second valueif a write event did not occur at the timestep.
 8. The storage drive ofclaim 6, wherein: the first constant is a decay rate constant; thesecond constant is an attack rate constant; and the first constant isequal to 1 minus the second constant.
 9. The storage drive of claim 1,wherein the write frequency ranking for the first logical address isbased on the average of the elapsed times of the write cycles.
 10. Thestorage drive of claim 1, wherein the write frequency ranking for thefirst logical address is based on a normalized average of the elapsedtimes of the write cycles.
 11. The storage drive of claim 1, wherein thelowest write frequency ranking of the first memory is greater than ahighest write frequency ranking of the logical addresses of the secondmemory.
 12. A method of operating a storage drive, wherein the storagedrive comprises a first memory and a second memory, wherein a writecycle lifetime of the second memory is less than a write cycle lifetimeof the first memory, and wherein each of the first memory and the secondmemory comprises solid-state memory, the method comprising: receiving afirst logical address from a host, wherein the host is separate from thestorage drive; determining a write frequency for the first logicaladdress, wherein the write frequency indicates how frequently data iswritten to the first logical address; based on the write frequency,determining a write frequency ranking for the first logical address,wherein the write frequency ranking is based on (i) a weightedtime-decay average of write counts, or (ii) an average of elapsed timesof write cycles; determining whether the write frequency ranking isgreater than a lowest write frequency ranking of logical addresses ofthe first memory; and if the write frequency ranking of the firstlogical address is greater than the lowest write frequency ranking oflogical addresses of the first memory, mapping the logical address withthe lowest write frequency ranking in the first memory to the secondmemory.
 13. The method of claim 12, wherein the mapping of the logicaladdress with the lowest write frequency ranking in the first memory tothe second memory includes mapping the logical address with the lowestwrite frequency ranking in the first memory from a physical address ofthe first memory to a physical address of the second memory.
 14. Themethod of claim 12, further comprising: if the write frequency rankingof the first logical address is greater than the lowest write frequencyranking of logical addresses of the first memory, mapping the firstlogical address to the second memory subsequent to mapping the logicaladdress with the lowest write frequency ranking in the first memory tothe second memory; and if the write frequency ranking of the firstlogical address is less than the lowest write frequency ranking oflogical addresses of the first memory, mapping the first logical addressto the second memory.
 15. The method of claim 12, further comprising, ifthe write frequency ranking of the first logical address is greater thanthe lowest write frequency ranking of logical addresses of the firstmemory, moving data at the logical address of the first memory with thelowest write frequency ranking to the second memory.
 16. The method ofclaim 12, wherein the write frequency ranking for the first logicaladdress is based on the weighted time-decay average of write counts. 17.The method of claim 16, wherein: the weighted time-decay average ofwrite counts is based on a sum of (i) a product of a first constant anda time averaged write count at a timestep, and (ii) a product of asecond constant and a write event value; the write event value is equalto (i) a first value if a write event occurred at the timestep, and (ii)a second value if a write event did not occur at the timestep; the firstconstant is a decay rate constant; the second constant is an attack rateconstant; and the first constant is equal to 1 minus the secondconstant.
 18. The method of claim 12, wherein the write frequencyranking for the first logical address is based on the average of theelapsed times of the write cycles.
 19. The method of claim 12, whereinthe write frequency ranking for the first logical address is based on anormalized average of the elapsed times of the write cycles.
 20. Themethod of claim 12, wherein the lowest write frequency ranking of thefirst memory is greater than a highest write frequency ranking of thelogical addresses of the second memory.